at the lowland forest at La Selva Biological Station, of
185 tree species studied, 90% produce fleshy fruits; of
those, approximately 50% are primarily bird dispersed,
13% bat dispersed, and the remainder dispersed by
other mammals, such as monkeys and agoutis.
A study of fruit dispersal of Casearia corymbosa
(a species of the willow family, Salicaceae) in Costa
Rica conducted by Henry Howe in 1977 recorded
21 species feeding on the tree’s fruits, none of which
really contributed to seed dispersal. These species
consumed the fruit at the tree and so were essentially
useless as seed dispersers. But one bird species, the
Masked Tityra (Tityra semifasciata; plate 10- 15), was
considered an efficient seed disperser. This thrush- size
black- and- white bird fed heavily on Casearia fruits
and regurgitated viable seeds at considerable distance,
well outside the seed shadow of the parent tree. Howe’s
work is one of many studies to demonstrate that
fruiting trees attract a diverse array of birds, mammals,
and reptiles (e.g., iguanas) but that not all of these
species, or even most of them, are suitable for efficient
dispersal of seeds.Fruits: The Evolutionary Stimulus for
Sexual Selection in Tropical BirdsCharles Darwin developed his theory of sexual
selection to account for why certain animal species,
ranging from beetles to birds and mammals (including
humans), show strong morphological differences
between the sexes. Why would selection act differently
between males and females? Among numerous bird
species, males are typically brighter in plumage (a
pattern termed sexual dichromatism) and often larger
than females. Why females have cryptic plumage
seemed an easy question to Darwin. The cryptic
female plumage aids in reducing the risk of discovery
by predators. But why are males so colorful? Adding to
this mystery was the fact that elaborately colored males
often augment their already gaudy selves by engaging
in bizarre and attention- getting courtship displays.
Sexual selection evolves through two pathways
operating separately or simultaneously. One is male–
male competition for access to females. This helps
account for why males tend toward larger body size thanGulpers and Mashers
Birds are selective about the size of the fruits they eat
and how they consume them. Species such as toucans,
aracaris, and toucanets pluck fruit, juggle it in the bill,
and then often reject it by dropping it. Large fruits are
particularly at risk of rejection and may be found scarred
by bill marks. Nathaniel Wheelwright hypothesized that
plants are under strong selection pressure to produce
small to medium- size fruits, as larger ones are rejected
by most bird species except those with the widest gapes.
Thus large fruits will tend to be selected by large birds
such as curassows and guans. Large fruits permit more
energy to be stored in the seeds, an advantage once
dispersal and germination have occurred.Studies by various researchers in Costa Rica indicated two
basic methods by which birds devour fruit. Anyone can
observe these methods in the field. Some birds (mashers)
mash up the fruit, dropping the seeds as they do, while
others (gulpers) gulp the fruit whole, subsequently either
regurgitating or defecating seeds (plates 10- 16– 17).
Mashers are mostly finches and tanagers, and gulpers
are toucans, trogons, and manakins. Mashers appear
more sensitive to taste than gulpers, showing a distinct
preference for fruits rich in sugars. Gulpers swallow fruit
whole and appear taste insensitive. Plate 10-^ 17. This Toco Toucan (Ramphastos toco) is plucking
a fruit that it will then gulp down whole. Photo by John
Kricher.Plate 10- 16. This Grayish Saltator (Saltator coerulescens) is an
obvious example of a masher. Photo by John Kricher.chapter 10 tropical intimacy: mutualism and coevolution 163